[jyl]

Can I get a check from you engineer types?

I am supporting two corners of the treehouse platform with lag screws into separate trees, and two corners with posts set in concrete.

The largest lag screw I can find is 1/2" diameter, ungraded.

My math is that the cross-section area of the unthreaded part of the lag screw (where the load will be) is 3.1416 x ( 0.25 in ^2 )= 0.196 sq in. I read that low-alloy steel has a shear strength of 45,000-50,000 lbs/sq in. Since these are ungraded zinc-plated steel screws from Home Depot, I assume shear strength only 30,000 lbs/sq in. I get that each screw should support, in shear, appx 8,800 lbs.

The maximum weight on the platform (including the platform, house, and as many adults as will be able to fit in the house, will be about 4,400 lbs.

So I am thinking 1/2" diam lag screw will be enough metal.

The screw will penetrate 6" into the tree trunk. The load will be perpendicular to the screw shank. I will add a second, similar lag screw about 6 feet higher on the trunk and run a steel cable to the platform, as a back-up support. I don't know how to calculate the holding power of a screw in wood, so am just "assuming" it will not pull out.

Critiques, corrections, warnings? Thanks.